No Heat

Hello Everyone,
I have a chevy blazer 2003 4.3 L V6 4door 4WD. I noticed a few weeks ago I had cold air blowing from the vents when the heat was on. as I drove to work I was watching the temp gauge it was at normal operating temp, just under 200 degrees and then began to rise up to about 230 then heat started coming from the vents and the temp dropped to normal. it went through this cycle several times as I drove to work. the first thing I thought to check was the coolant level witch brings me to problem #2, I can't get the hood open. I pull the lever I here a click but the hood will not pop when the lever is released. I had a coolant leak about 3-4 months ago I replaced the radiator, water pump and thermostat I have not had any visible coolant leak since. I only had that issue one time where it cycled through hot air and cold air otherwise it runs about 192 degrees. I am sorry if this post is somewhat premature but Since I can't even get the hood open to check the basics my imagination is running wild with spending lots of money and lost weekends spent under the hood of my truck. any help would be greatly appreciated

Chances are the latch is rusted and not releasing......

Have someone repeatedly pull on the lever, while with one hand you pull up lightly on the hood and with the other, use the palm of your hand to hit the top of the hood in the latch area....when it opens, lube the crap out of that latch......

Either two things....low on coolant(still have a leak somewhere), or your heater core is partially plugged.....feel the inlet and outlet hoses......if the inlet is much hotter, try reverse flushing the heater core....

on my hood, i pull the latch, then with a closed fist, smack it just above the latch, and it pops open.

Blazers have a heater core problem in design. They are installed in the dash laying down flat instead of standing up. They clog very easily. There is a Youtube video that shows how to flush your unit with a simply made 3/4" garden hose and a 1/2" garden hose. I made one in about 5 minutes. You simply plug up the garden hose to one pipe located on the fire wall under hood and hook up the other garden hose to the other pipe (one pipe is a 3/4 and the other is a 1/2"). Turn on the water all the way and let run until clear water is going out the outflow hose. Turn off water and a vacuum forms that forces more stuff out. Turn you water on again for a couple minutes, turn off again to force again. Then do a third time. Takes about 5 - 10 minutes and saves going to the garage where they will tell you that you need a coolant system flush that is way overpriced. Haven't had to go to the garage for this problem in about 4 years now and only do the core flush once a year whether needed or not. Heat has run great ever since.
There was also a problem with class action lawsuit on the intake manifold gasket design by Chevrolet on the Blazer. DexCool would cause it to basically rot. The thing would foul and leak dexcool causing the temp gauge to go up and down. The class action is over now and I had to repair my 2000 blazer at my expense. Cost me 700 bucks at the dealer as it was a little too much for me to take on replacing the gasket. But try flushing your core first with above.
Hope this helps.